Recent advances in long-term preservation of viable gonadal tissues from Carnivora

The sustainability of domestic and wild carnivore populations is increasingly dependent on advanced reproductive technologies, among which long-term preservation of viable gonadal tissues has emerged as an essential tool. Ovarian and testicular tissues are reservoirs of stem cells, germ cells at various developmental stages, and somatic cells that can be used in assisted reproduction. Studies on gonadal tissue preservation in domestic cats and dogs are mainly used as models to increase options for genetic rescue of wild counterparts, where natural breeding opportunities might be limited during reproductive life or when individuals die before sexual maturity or outside the breeding season. Mainly based on studies published since 2020, the objective of the review is to provide an overview of the most promising progress in gonadal tissue preservation in domestic and wild carnivores and propose research directions for the coming years. Recent studies have defined better conditions for cryoprotectant exposure, freezing protocols, and thawing-warming methods that lead to good tissue structure and viability after preservation. In general, there are less reports and fewer in-depth studies in domestic dogs compared to domestic cats. In wild carnivores, the opportunistic nature and simplicity of the sample collection result in more studies on testicular tissues. Over the recent years, the integration of molecular technologies, such as transcriptomics, has offered novel avenues for improving tissue preservation protocols and long-term storage. Despite these advancements, significant gaps remain to be filled in the characterization of species-specific preservation conditions and in the optimization of tissue xenografting or germ cell culture systems after thawing-warning to produce matured gametes. As an alternate option to freezing temperatures, dehydration for possible storage at non-freezing temperatures is an approach that is currently developed in the domestic cat model and that should be pursued to address logistical and financial constraints of cryo-storage in wild carnivore conservation.